U.S. patent number 5,489,980 [Application Number 08/439,436] was granted by the patent office on 1996-02-06 for apparatus for rapid and accurate analysis of the composition of samples.
Invention is credited to Michael Anthony.
United States Patent |
5,489,980 |
Anthony |
February 6, 1996 |
Apparatus for rapid and accurate analysis of the composition of
samples
Abstract
An apparatus comprising two light sources, a composite detector,
a fixed grating, two independent slits and a mask with a
multiplicity of slits analyzes the spectral composition of samples
rapidly and accurately and can transmit such information to other
locations by modem. A first light source produces a spectrum with
broad spectral range, a second light source produces a spectrum
with multiple sharp spectral features. The first and second light
sources are used to produce a sample spectrum and a reference
spectrum respectively. A portion of the light from each of the two
sources is used to calibrate the intensity of the instrument at
each wavelength measurement. Rapid scanning is achieved by
continuous multiplexing of each wavelength of light to the detector
using a rotating mask with a multiplicity of slits. Continuous
wavelength calibration is achieved by using the reference spectrum
to encode a wavelength scale as spectrum is acquired. The spectral
data can be transmitted by the said apparatus to other locations by
modem. The said modem enables a multiplicity of the said apparatus
to be used at various locations to perform a common analysis
function. For example, a city wide medical network of analyzers may
be set up to communicate with a central data base, where analyses
on clinical assays may be performed by powerful dedicated
computers. In another example, a network of the said apparatus may
be set up in an integrated manufacturing environment such as a
tobacco manufacturing plant or pharmaceutical manufacturing plant,
to accumulate data at several points in the manufacturing process.
The apparatus, may be used to rapidly scan and analyze discrete
moving samples for composition analyses, density determination,
moisture determination, color, and surface uniformity.
Inventors: |
Anthony; Michael (Coral
Springs, FL) |
Family
ID: |
25316004 |
Appl.
No.: |
08/439,436 |
Filed: |
May 11, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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853424 |
Mar 18, 1992 |
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Current U.S.
Class: |
356/308;
356/328 |
Current CPC
Class: |
G01J
3/06 (20130101); G01J 3/2889 (20130101); G01J
2003/2866 (20130101) |
Current International
Class: |
G01J
3/00 (20060101); G01J 3/28 (20060101); G01J
3/06 (20060101); G01J 003/06 (); G01J 003/20 ();
G01J 003/10 () |
Field of
Search: |
;356/305,308,320,326,328 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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83922 |
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Apr 1986 |
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JP |
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883471 |
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Nov 1961 |
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GB |
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787909 |
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Dec 1980 |
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SU |
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Primary Examiner: Evans; F. L.
Attorney, Agent or Firm: Holland & Knight
Parent Case Text
This is a continuation of application(s) Ser. No. 07/853,424 filed
on Mar. 18, 1992 abandoned.
Claims
I claim:
1. An apparatus for the spectral analysis of the composition of a
sample, said apparatus comprising:
a first light source optically coupleable to the sample for
illuminating the sample;
a second light source;
A spectrograph having a first entrance aperture optically coupled
to the sample for collecting at least a portion of the light
dispersed from the sample and a first second entrance aperture
optically coupled to said second light source, each of said first
and second apertures lying on a common Rowland circle, said
spectrograph also having a diffraction grating positioned to
receive light from said first and second entrance apertures and to
disperse a combined spectrum thereof onto a first photo-sensitive
transducer, said first photo-sensitive transducer having a first
photo-sensitive surface at least a portion of which is located
substantially on said Rowland circle; and
a movable mask optically interposed between said grating and said
transducer, said mask including a plurality of openings arranged to
intermittently expose said first surface to said spectrum as said
mask is moved.
2. The apparatus of claim 1 wherein said mask is movable rotatably
and wherein said openings are disposed radially about an axis of
rotation of said mask.
3. The apparatus for claim 1, further comprising a light intensity
control for controlling the intensity of at least one of said first
light source and said second light source.
4. The apparatus of claim 1, wherein said first light source
comprises a source for emitting light having a wide wavelength
range and substantially no prominent spectral peaks and said second
light source comprises a source for emitting light which includes a
plurality of prominent spectral peaks.
5. The apparatus of claim 4 wherein said wide wavelength range is a
range primarily within the visible near infrared range of the
electromagnetic spectrum and said prominent spectral peaks of said
second light source include peaks lying in the visible and near
infrared region of the electromagnetic spectrum.
6. The apparatus of claim 1 wherein said diffraction grating
comprises a concave holographic grating.
7. The apparatus of claim 3, further comprising:
a second photo-sensitive transducer and optical path means for
conducting to said second photo-sensitive transducer a portion of
said light dispersed from said sample as well as a portion of light
emitted from said second light source, said second photosensitive
transducer being coupled to said light intensity control for
controlling said intensity of said at least one of said first and
second light sources.
8. The apparatus of claim 7 wherein said first and second
photosensitive transducers are formed on a common substrate.
9. The apparatus of claim 1 further comprising means for effecting
relative movement along said Rowland circle between said first and
second entrance apertures.
10. An apparatus for the spectral analysis of the composition of a
sample, said apparatus comprising:
a first light source optically coupleable to the sample for
illuminating the sample with first light having a wide wavelength
range and substantially no prominent spectral peaks;
a second light source capable of emitting second light which
includes a plurality of spectral peaks lying within said wavelength
range, at least some of said spectral peaks being of an intensity
greater than that of said first light source; a spectrograph having
a first entrance aperture optically coupled to the sample for
collecting at least a portion of the light dispersed from the
sample and a second entrance aperture optically coupled to said
second light source, each of said apertures lying on a common
Rowland circle, said spectrograph also having a diffraction grating
positioned to receive light from said first and second entrance
apertures and to disperse a combined spectrum thereof onto a first
photo-sensitive transducer having a first photosensitive surface at
least a portion of which is located substantially on said Rowland
circle, and a second photo-sensitive transducer;
a movable mask optically interposed between said grating and said
first transducer, said mask including a plurality of openings
arranged to intermittently expose said first surface to a portion
of said spectrum as said mask is moved;
optical path means for conducting to said second photo-sensitive
transducer a portion of said first light dispersed from said sample
as well as a portion of said second light emitted from said second
light source; and
computing means interfaced to said first and second transducers for
determining a wavelength scale based on the wavelengths of said at
least some of said spectral peaks.
11. The apparatus of claim 10 wherein said first and second
photosensitive transducers are formed on a common substrate.
12. The apparatus of claim 10 wherein said computing means further
includes means for extracting a spectrum of the sample based on
said combined spectrum, the spectral positions of said at least
some of said spectral peaks and an intensity correction factor,
said intensity correction factor being correlated to changes in the
intensity of said portion of said first light dispersed from said
sample and said portion of said second light emitted from said
second light source.
13. The apparatus of claim 12 further comprising means coupled to
said computing means for transmitting and receiving data to and
from a remote location.
Description
FIELD OF THE INVENTION
This invention relates to the field of spectroscopy.
BACKGROUND OF INVENTION
General
Spectroscopy is a method of measurement of the absorption and
emission of electromagnetic waves by substances. When polychromatic
light is directed at a sample, the sample attenuate different
wavelengths of the electromagnetic spectrum in a specific way. This
allows different materials to be readily identified by their
spectral signature. For example, helium gas was first discovered by
identification of it's unique signature in the sun's spectrum. The
near infrared (NIR) region of the electromagnetic spectrum is
particularly useful for analyzing samples of complex
composition.
The said NIR region lies between the visible region and the
infrared region of the electromagnetic spectra extending from 700
to 2600 nanometers. The bulk of NIR spectrum arise as a result of
vibrational overtone stretches of the OH, NH, and CH groups of
chemical bonds which are present in natural and man made products
such as blood serum, plastics, tobacco and food products.
Present day near infrared spectroscopy is an analytical technique
of wide applicability and requiring minimum sample preparation.
Sampling can be performed without contact, generally requires no
sample pre-treatment or separation techniques. The major advantage
of NIR spectroscopy lies in its large signal to noise ratio, making
it possible to analyze even trace constituents accurately, without
the use of expensive Fast Fourier Transform analyzers (FFT).
Furthermore, modern statistical analytical methods such as expert
chemometric models, coupled with specialized sampling accessories
such as fiber optic probes and flow cells, allow a wide variety of
materials and mixtures to be readily analyzed in the NIR region of
the spectrum. Expert chemometrics modelling allows multicomponent
analysis of complex mixtures, or matrices, based on the knowledge
of the underlying spectroscopy. Special algorithms allow an
analyzer to measure qualities such as shelf life, tackiness, taste,
and uniformity in addition to sample composition, in a non-invasive
manner.
Minimal sample preparation and rapid analysis yield strong
advantages over the conventional laboratory methods of chemical
analysis. Laboratory methods of chemical analysis, though very
accurate, can be labor intensive and time consuming. These
drawbacks coupled with the possibility of sample contamination by
human errors make NIR spectroscopy a desirable analytical tool.
However, spectroscopic analysis methods can only yield results that
are as accurate as the primary calibration method used to calibrate
the analyzer. But by continuous calibration using all possible
sample types within a measurement, errors can be minimized to yield
a high degree of confidence in the method.
There are many areas in which NIR spectroscopy is successfully
applicable. These include: the tobacco industry, pulp and paper
industry, petrochemicals, biomedical, pharmaceutical, foods and
beverages. The bio-medical industry is a particularly interesting
field of application. NIR spectroscopy has been very successful as
a fundamental analytical technique for effecting biomedical assays.
Multiwavelength NIR spectroscopy, combined with sophisticated data
analysis technics based on multivariate statistics, offer an
attractive alternative to conventional analysis methods. The recent
developments in Neural Network technology has added the advantage
of nonlinear modelling for analysis of samples exhibiting high
variability.
Instrumentation
Current designs of near infrared analyzers involve either a
stationary grating or a moving grating. Moving grating instruments
are slow cumbersome and susceptible to mechanical errors due to the
large mass of the grating. They have the advantage of using a
single detector element. The speed of such instruments may reach 10
scans per second, limiting their use to single applications when
the number of scans needed to make significant statistical
measurement is greater than the instrument scanning speed. Landa et
al discloses such an analyzer in U.S Pat. No. 4,540,282. On the
other hand, the stationary grating instruments are fast but not as
accurate as the moving grating instruments. Their speed is
primarily due to the use of diode array detectors with well over
200 separate elements, which simultaneously measure the spectrum
energy levels. Each element measures a fixed segment of the
spectrum produced by a spectrograph, so that the resolution of the
spectrum recorded depends primarily on the number of diodes used.
Detector array elements tend to have slightly different drift
characteristics, thereby requiring expensive electronics for making
corrections to the spectrum. Diode array systems of current designs
are very expensive and still not fully developed, making the
currently available technology too expensive for most applications
where multiple analyzers may be required.
State of the art wavelength and intensity calibration technics
So far, state of the art analyzers cannot effectively compensate
for rapid variations in the measurements resulting from the harsh
conditions of a manufacturing environment without dedicating
considerable time to measuring separate "reference" scans. This
limits the rate of actual sample analyses considerably. The user
must trade accuracy of measurement of sample scans for speed. The
wavelength referencing methods used by these analyzers, depends on
sequentially comparing a reference spectrum with the sample
spectrum to obtain corrections for the said sample spectrum. U.S
Pat. No. 5,020,909 to Landa et al discloses such a method. In the
Landa patent, the reference spectrum is sequentially convalued with
the sample spectrum and then extracted by mathematical means before
correction of said sample spectrum is done. The technique employed
by Landa et al. depends on time sharing the analyzer between the
said reference scans and sampling scans. The extracted reference
spectrum is then compared to the known reference spectrum in order
to obtain the corrections for the wavelength shifts that may have
occurred during data acquisition on said reference spectrum. In
fact, the reference spectrum must be corrected prior to correcting
the sample spectrum. The correction factors for said reference
spectrum are assumed to be valid for the sample spectrum correction
also. The method also depends on convolution of the reference
spectrum and the sample spectrum.
The process of convolution is time consuming and presents certain
difficult mathematical problems. The general mathematical
definition of convolution is as follows:
If a function f(t) is convalued with g(t), then ##EQU1## where * is
the convolution operation and x and t are independent
variables.
It is thus clear that the process of convolution involves
continuous shifting of one spectrum with respect to another and
integrating the product of the overlapping spectra. The light is
first attenuated by the reference material and then attenuated by
the sample. Although Landa et al. does not outline the means of
performing the convolution it is clear that several possible
outcomes could result from a mathematical deconvolution process.
Also the spectrum of the said reference material could in fact be
superficially affected by pathlength changes or sample chemistry
changes in the reference material. The Landa et al. disclosure does
not distinguish between the effects of instrument variation during
the acquisition of said reference spectrum from the effects of
instrument variation during the acquisition of the sample spectrum.
The Landa disclosure assumes that the errors found by comparing the
"deconvoluted" spectrum of the reference material to a known
reference spectrum of the said reference material are identical to
the errors that will have occurred during the acquisition of the
said sample spectrum. This may not be the case for samples with
high variability. There are two examples for which the said Landa
et al. disclosure cannot produce accurate results. Firstly, if the
sample were taken to be a spectroscopically clean stable sample in
an evacuated chamber with no mechanical or thermal stresses,
variations in the reference material, sampling accessories,
analyzer electronics and optical components would result in
variations in the deconvoluted spectrum of the said reference
material from the known spectrum of the said reference material.
The Landa et al. patent discloses a method which will employ these
measured variations to correct the spectrum of an essentially
spectroscopically clean sample, thereby defeating the purpose of
the method. Secondly, if the sample was on a fast moving conveyor
belt, spectral shifts caused by one portion of the said sample
would result in corrections of the spectrum of another portion of
the said sample spectrum. This type of correction is referred to as
cross correction. The method disclosed by Landa et al. is therefore
not suitable for fast moving samples or spectroscopically clean
samples. The present invention addresses the need for high speed
accurate scanning and analyses of samples in industrial
environments particularly for sorting application, without the
drawbacks of diode array based analyzers, moving grating based
analyzers and cross correction. A novel wavelength referencing
method is disclosed by the present invention, which addresses the
need for simultaneous correction of a sample spectrum without need
of separate reference scans.
SUMMARY OF INVENTION
The apparatus of this disclosure consists of a first broad spectrum
light source; a second reference light source with multiple sharp
spectral peaks and greater intensity than the first light source; a
spectrograph with two separate entrance slits, a concave
holographic diffraction grating and a composite light sensitive
detector element with first and second detection surfaces; a
motorized mask with multiple slits; and electronic circuits. Rapid
scanning is achieved by using a stationary optical arrangement for
the spectrograph and uniform speed rotation of the mask in front of
the detector element to multiplex each wavelength of the spectrum
unto the first sensitive surface of the detector element. The
spectrograph is constructed with a first entrance slit and a second
entrance slit symmetrically placed on the "normal plane" of the
grating at equal angular orientations from the "grating normal" of
the grating. The "grating normal" of the grating is the
hypothetical line which forms the axis of cylindrical symmetry of
the concave grating. The "normal plane" of the concave grating is
the horizontal plane in which lies the center of the concave
grating and the grating normal. The normal plane is perpendicular
to the first and second entrance slits, so that the slits are
vertically oriented. The grating disperses the incident light from
the sample into a "sample spectrum" of finite spatial distribution
at a fixed focal plane of the grating. The spectral lines are
vertical on the focal plane, and each spectral line is centered on
the normal plane of the grating. The focal plane of the grating is
perpendicular to the normal plane of the grating and located at a
finite distance from the grating.
The light from the sample is directed at a first entrance slit of
the spectrograph, from which thereof it is made incident on the
surface of the concave holographic grating of the said spectrograph
at a fixed incident angle .alpha. with respect to the grating
normal.
The light from the reference light source is directed at a second
entrance slit of the spectrograph and made incident upon the
grating at an angle .beta. to the normal of the grating. In the
uncalibrated configuration of the apparatus, the angle .beta. is
equal to the angle .alpha..
In a calibrating mode, the angle .beta. can be varied by moving the
second entrance slit of the spectrograph using a motor and
electronic controls, or by manual rotation of a knob on the
apparatus. The light from the said second entrance slit is also
dispersed into a "reference spectrum" at the focal plane of the
grating. The spatial distribution of the "reference spectrum" can
be shifted with respect to the spatial distribution of the "sample
spectrum" by changing the angle .beta. of the second entrance slit
with respect to the grating normal within the normal plane of the
grating. The energy distribution of the grating is such that it
will focus the light from the entrance slits, at the same focal
plane with respect to the grating normal when the entrance slits
are symmetrically placed with respect to the grating normal
(.alpha.=.beta.), and centered on the normal plane. The position of
the second entrance slit relative to the first entrance slit can be
adjusted in order to shift the reference spectrum with respect to
the sample spectrum. The position of the first entrance slit is
fixed with respect to the concave grating. This also makes the
position of the sample spectral distribution on the focal plane
fixed. The motorized mask is positioned at the focal plane of the
grating to completely block off the spectra from reaching the first
sensitive surface of the detector element. The motion of the masked
disc allows each slit to "chop" the entire spectral region
sequentially by allowing a single wavelength at a time through the
slit as it moves across the spectrum, to be measured by the first
sensitive surface of the detector element. A first light guide
member collects a fixed portion of the light from the sample for
intensity calibration measurements by the second sensitive surface
of the detector element. A second light guide member collects a
fixed portion of the reference light for intensity calibration
measurements by the second sensitive surface of the detector
element. The second sensitive surface of the detector is
continuously exposed to the superimposed light of the said first
and second light guide members. The second sensitive surface of the
said detector produces a continuous analog "intensity calibration"
electrical signal which corresponds to the combined intensity of
the light emitted by the first and the second light guide
members.
An Analog to Digital Converter (ADC) is used to convert the said
analog "intensity calibration" electrical signal into digital
information at a fixed sampling rate. The digitized "intensity
calibration" electrical signal is stored in the memory of a
microprocessor for intensity calibration at each wavelength point
of the measured spectrum.
The superimposed spectra of the sample light and the said reference
light source is "chopped" by each slit of the mask in order to
allow the intensity of each wavelength of light of the superimposed
spectrum to be measured by the first sensitive surface of the
detector element. The design of the mask is such that only one slit
of the mask is exposed to the superimposed spectra at any given
instant in time. The detector element produces an analog "spectral
intensity" electrical signal whose strength corresponds to the
intensity of the superimposed spectra at each point of the spectral
distribution as measured by the first sensitive surface of the
detector element. The analog "spectral intensity" electrical signal
is digitized at a fixed sampling rate by another Analog to Digital
Converter (ADC) and stored in the memory of the microprocessor as
spectral information. The microprocessor controls the two ADCs and
synchronizes the sampling of the two signals produced by the
detector element. The microprocessor determines the location of the
spectral, peaks of the reference light source with respect to the
number of A to D sample counts and uses the known spectral features
of the reference light source to automatically generate a
wavelength scale for the measurements.
A calibration routine allows the known spectral peaks of the
reference spectrum to be used to map the wavelength scale of the
sample spectrum. The wavelength region of interest, for measuring
the sample spectrum is chosen by electronically or manually
positioning moving the second entrance slit to position the
reference spectrum evenly across the spatial spectral distribution
of the sample spectrum in the region of interest on the focal plane
of the superimposed spectra. This manipulation of the reference
spectrum can be compared to positioning an optical measuring rod
across the sample spectrum in the region of interest in order to
generate a wavelength scale for the region. The properties of such
a hypothetical measuring rod would be correlated to the optical
properties of the spectrograph. Thus if the spectrograph causes any
change in spatial distribution of the sample spectrum, the same
changes will automatically occur in the reference spectrum,
nullifying the effects of such a change. The light intensity of the
spectrum at each point of the spectral distribution is
automatically determined by the microprocessor using the combined
digital signals from the two sensitive surfaces of the detector
element. The microprocessor uses the digitized intensity
calibration electrical signal obtained from the second sensitive
surface of the detector element to obtain accurate sample spectral
intensity information at each wavelength. The method of Continuous
Wavelength Calibration disclosed herein allows the wavelength scale
to be dynamically imprinted on the said detector element at each
and every scan cycle of the apparatus. Because there is no time
difference between a sample measure scan and a wavelength reference
or calibration scan, the wavelength scale generated for the
measurement is very accurate. The potential errors introduced by
conventional methods involving the use of an encoder to determine
the spatial location of the rotating mask in order to define a
wavelength scale are completely eliminated by the present
invention, since the reference wavelength scale so generated by
this invention is ultimately determined by the spectrum of the
second reference light source and not by an independent entity such
as an encoder. A variation in the properties of the grating cannot
affect the said wavelength scale generated by the method here
described, since such a variation is automatically corrected by
equal variation in the reference spectrum scale. Variation in the
rotational speed of the rotating mask is accounted for by the
present invention, since the rotational speed of the mask can be
determined by knowledge of the location of the second reference
light source spectral peaks and the time difference separating the
peaks as measured by an electronic clock. The method of Continuous
Intensity Calibration disclosed herein automatically accounts for
any external variations in light intensity caused by either ambient
light, temperature fluctuations of sample, discontinuities in the
sample, sudden changes in the voltages of both the reference light
source and the primary light source of the said apparatus.
A modem installed in the apparatus, allows the apparatus to receive
and transmit data using existing telecommunication networks.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of the apparatus of the present
invention.
FIG. 2 is a block diagram of the electro-optical arrangement of the
apparatus of FIG. 1.
FIG. 3 is a simplified drawing of the optical arrangement of the
apparatus at the exit port in reflectance mode.
FIG. 4 is a simplified drawing of the optical arrangement of the
apparatus at the exit port in transmission mode.
FIG. 5 is a representative drawing of the fiber optic light guide
coupling of the apparatus.
FIG. 6 is a representative drawing of the optical arrangement of
the instrument in the direct mode.
FIG. 7 is a simplified drawing of the mask and its spatial
relationship to the detector element.
FIG. 8 is a frontal and side view of the mask with the radial slit
pattern.
FIG. 9 is a flow diagram of a typical scan cycle of the
apparatus.
FIG. 10 shows graphs of the superimposed reference and sample
spectrum and the extracted sample spectrum respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENT
While this invention is satisfied by embodiments in many different
forms, there will herein be described in detail the preferred
embodiment of the invention, with the understanding that the
present disclosure is to be considered as exemplary of the
principles of the invention and is not intended to limit the
invention to the embodiment illustrated and described. The scope of
the invention will be measured by the appended claims and their
equivalents.
The apparatus 10 comprises: a first light source 101, a second or
reference light source 102, collimating lens 103, collimating lens
118 (FIG. 6), a spectrograph 104, a motorized mask 200 with a
geometric pattern of slits, a photo detector 201 with two separate
detection surfaces 201a and 201b electronics 700 shown in FIG. 1, a
modem 800 and a microprocessor 900. The apparatus 10 is designed to
capture a complete spectrum of a sample with high resolution and
high speed using a stationary electro-optical arrangement and
rotating mask 200. The spectrograph 104 includes a concave
holographic grating 106, a first fixed entrance slit 105a and a
second adjustable entrance slit 105b. Grating 106 disperses light
from each of the said entrance slits 105a and 105b into two
distinct flat field spectra 107a and 107b with uniform wavelength
distributions. The relative positions of the said entrance slits
105a and 105b can be adjusted dynamically to generate spatially
independent spectra 107a and 107b of the reference light source and
the sample respectively, or superimposed spectra 107d of the
reference light source and the sample spectrum. First light source
101 produces light with a broad spectrum in the spectral region of
interest. Second or reference light source 102 produces light with
multiple sharp spectral features in the spectral region of
interest. Light from the said first light source 101 is collimated
into beam 110 by the collimating lens 103. The beam 110 is directed
at an exit port 112 of apparatus. Exit port 112 of the apparatus 10
can be fitted with either a sample holding device 12 or a fiber
optic light guide coupling 13. The construction of the said fiber
optic wave guide coupling 13 is shown in FIG. 5.
When apparatus 10 is fitted with a sample holder 12, it can be
described as being adapted for use in a direct mode. When apparatus
10 is fitted with a light guide coupling 13, it can be described as
being adapted for use in remote mode.
Direct Mode
In the direct mode, light 110 from the said first source 101 is
made to strike a target sample 14 at an incidence angle of
45.degree. in the horizontal plane. The incident light is either
transmitted through sample 14, or reflected in a specular fashion
from sample 14. For transmission measurements in the direct mode, a
highly reflective surface 114a such as a mirror with a broad
spectral range is placed on sample holder 12, so that the
transmitted light is redirected into collection optics 15 of the
apparatus 10 at 90.degree. to incident beam 110a in the horizontal
plane. The horizontal plane will here be take to be the
hypothetical plane on which lies the center of the grating, the
grating normal and the center of the two slits. For reflectance
measurements on a sample, a diffuse reflector 114b such as ceramic
is placed on sample holder 12, so that the specular reflectance of
the said sample is redirected into the collection optics 15 of
apparatus 10. This configuration is suitable for slurties, powders,
colloids and solids. Such materials may include tobacco, pulp and
paper, resins or pastes.
Remote Mode
FIG. 5 is a representative drawing of the fiber optic wave guide
coupling of the apparatus at the exit port in Remote mode.
In Remote Mode, exit port 112 is fitted with a light guide coupling
13. The light 110 from the first source is focussed by the said
coupling 13 into a first end of a light guide 16. Light guide 16 is
constructed of materials such as fuzed quartz or fuzed silica,
suitable for transmission of the desired spectral range. The light
transmitted by light guide 16 is emitted at a second end and
directed to a remote sampling device 18 such as a fiber optic probe
or a flow cell. Such devices are described in U.S. Pat. No.
5,044,747 to Anthony et al, and U.S. Pat. No. 5,044,755 to Anthony
et al. The light 110 may also be directed into a light multiplexing
device to provide light to a plurality of sampling devices such as
18.
U.S. Pat. No. 4,989,932 Anthony et al describes such a multiplexing
device.
Sampling device 18 is designed to direct the light 110 at a target
sample 19 in order to make reflectance or transmission
measurements. The probes or flow cells may be constructed to suit
the conditions that the sample must maintain during measurements. A
second light guide 17 collects the reflected light from the said
sample at a first end, and transmits the light through a second end
attached to collection optics 15 in apparatus 10.
FIG. 6 is a representative drawing of the optical arrangement of
the apparatus in the direct mode.
In both the direct and remote modes, collection optics 15 focus a
portion of the collected light into a beam 115a at a first
stationary entrance slit 105a of spectrograph 104. The remaining
portion of the beam 115a is collected by the first end of a light
guide 116a. While light guides 16 and 17 (FIG. 5) may be detached
from the apparatus, light guide 116a is a permanent but replaceable
component of apparatus 10.
Slit 105a is used to reduce the incident beam 115a into a line
source 117a for spectrograph 104. Slits 105a and 105b lie in a
vertical plane to the normal plane of grating 106. The said normal
plane of grating 106 is the hypothetical horizontal plane on which
lies on the axis of cylindrical symmetry of the grating 106. The
grating normal is the hypothetical axis of cylindrical symmetry of
grating 106. The said normal plane is perpendicular to entrance
slits 105a and 105b which are vertically oriented with respect to
the said normal plane. Grating 106 disperses the incident light
from the sample into a "sample spectrum" of finite spacial
distribution at a fixed focal plane 107c (FIG. 7) whose spatial
location is determined by grating 106. The focal plane 107c of
grating 106 is perpendicular to the normal plane and located at a
finite distance from grating 106. A line source increases the
wavelength resolution of the diffracted light from a spectrograph
by resolving the said spectrum into imaged lines of different
wavelength with about the same spacial width as the said slit. It
is desirable to make all slits of the apparatus 10 as small as
possible in order to increase the wavelength resolution of the
apparatus. The light 117a from slit 105a is made to fill grating
106 of spectrograph 104. Grating 106 disperses light 117a into a
spectrum 107a. The spectrum 107a lies on the Rowland circle 104a of
spectrograph 104. The Rowland circle 104a is the circle on whose
circumference lies the apex of grating 106, its spectral focus and
the center of slits 105a and 105b. The Rowland circle 104 a of the
said spectrograph 104 also lies on the normal plane of grating 106.
The light from the said second reference light source 102 is
collected and focussed into beam 110b by optical lens 118 at a
second adjustable entrance slit 105b of spectrograph 104. A small
portion of the light from reference light source 102 is collected
by the first end of a light guide member 116b.
The design of the spectrograph 104 is such that the second entrance
slit 105b is held by an adjustable stand 105c which also lies on
said Rowland circle 105c. The adjustable stand 104bcan be moved by
a motor 901 (FIGS. 1 and 8) along a path which lies on the
circumference of the Rowland circle 104a of spectrograph 104 either
manually or electronically by means of microprocessor 900 and drive
control electronics 700 as indicated in FIG. 1. The light 117b from
the slit 105b is also focussed by grating 106 into a spectrum 107b
at the said focal plane 107c. of grating 106. The spectra produced
by light beams 117a and 117b are independently focussed at the
common focal plane 107c to form a superimposed spectrum 107d. Slits
105a and 105b are cut on replaceable rectangular opaque masks 108a
and 108b respectively. Slits 105a and 105b have fixed heights and
widths. The height and width of the said slits may be changed to
increase or decrease the spectral resolution of the apparatus, by
replacing the masks 108a and 108b with ones having the required
slit dimensions. The preferred width of the slits 105a and 105b can
vary from 50 microns to 200 microns, with a fixed height of 12 mm.
Mask 200 also lies on the focal plane 107c, and is positioned to
completely block of the said superimposed spectrum 107d from
passing through to detector 201. Slits 202a of mask 200 act as the
only passageways for the superimposed spectrum 107d to detector
201. The slits 202a are made small enough to allow only a small
portion of spectrum 107d to pass through them. By moving a slit
202a across the spectrum 107d, the detector 201 can make intensity
measurement at each respective wavelength of the spectrum 107d.
In the preferred embodiment of this invention, mask 200 is
constructed from a thin rigid disc of radius 50 mm and thickness of
about 0.2 mm. A regular radial pattern of 12 equal slits 202a, 12
mm in length and 50 microns wide are cut radially from the
circumference. Mask 200 is replaceable, and may be constructed with
any number of slits with either a regular pattern or specially
designed patterns to serve special purposes. Detector 201 used for
this configuration is constructed with a first sensitive surface
201a and a second sensitive surface 201b respectively, on the same
substrate. Each sensitive surface of the said detector 201 is
capable of separate measurement of light intensity and separate
analog electrical signal outputs. Detector 201 may also be
constructed as a "self compensating" detector, so that the two said
analog signals may be combined electronically by a light intensity
control circuit 100 into a single analog electrical signal
representative of a "corrected" intensity spectral signal at any
given instant in time. It is advantageous to manufacture the two
sensitive surfaces 201a and 201b on the same substrate, because a
common substrate eliminates differences in physical characteristics
between the two sensitive surfaces of the detector, so that thermal
stresses or mechanical stresses will be the same for both detection
surfaces. Detector 201 has a height of 500 microns and a total
length of approximately 30 mm divided so that the first sensitive
surface 201a of detector 201 has a length of 25 mm and the second
sensitive surface 201b of detector 201 has a length of 5 mm.
Detector 201 is placed as close as possible to mask 200, with the
said first sensitive surface 201a positioned within the longest
cord segment between any two slits and the said second sensitive
surface 201b extending tangentially from the mask 200 as shown in
FIG. 8. As shown in FIG. 7, the sensitive surfaces 201a and 201b of
detector 201 lie on a plane parallel to focal plane 107c
immediately behind the mask 200 when viewed from the direction of
grating 106. Detector 201 can be made to measure any portion of the
spectrum 107d by rotating a particular slit 202a of mask 200 to
that portion of the spectral distribution. The axis of rotation of
mask 200 is perpendicular to the extended focal plane 107c and
located above spectrum 107d as shown in FIG. 7. The center of the
horizontal length of the first sensitive surface 201a of detector
201 is positioned at the center of the spectral distribution 107d
tangential to the Rowland circle 104a. The sensitive surfaces of
detector 201 are also perpendicular to and centered on the normal
plane of grating 106. Mask 200 is driven by stepper motor 300. The
step rate of motor 300 is accurately controlled by drive control
electronics 700. The angular position of mask 200 is determined by
optical encoder 600. Encoder 600 ensures accurate speed control of
motor 300 and provides a means of determining the direction of
rotation. The second ends of light guides 116a and 116b are
permanently fixed at the second sensitive surface 201b of detector
201, to continuously illuminate the sensitive surface 201b.
Detector 201 is constructed of materials that are suitably chosen
for the-wavelength range required. Such materials include indium,
gallium arsenide and lead sulphide. Such detectors are readily
available from several manufacturers such as Mitsubishi Inc. of
Japan, Hamamatsu Photonits Inc. of Bridgewater N.J.
The rotation of the mask 200 across the sensitive surface of
detector 201 allows spectrum 107d to pass through each slit 202a
respectively. The spectrum is chopped by each slit 202a so that
only a small region of the said spectrum may pass through any given
slit at any given time. This way, the detector 201 can be made to
measure only a small single wavelength band of the spectrum 107d at
any instant in time. Continuous rotation of the mask 200 allows the
entire spectrum to be measured a single wavelength at a time. Only
one slit 202a passes the spectral plane 107c at any given instant
in time. Detector 201 generates an analog signal whose strength is
proportional to the intensity of the light incident upon it. The
signal is continuously sampled at a fixed rate and digitized by the
analog to digital converter (ADC) 701 shown in FIG. 2.
CONTINUOUS SIMULTANEOUS WAVELENGTH CALIBRATION AND INTENSITY
REFERENCE METHOD
WAVELENGTH CALIBRATION MODE (WCM)
Before apparatus 10 can be used to acquire spectral data it must be
calibrated to recognize the wavelength scale for the measurements.
This is done to define the wavelength region for the sample
spectrum measurement, and to create an accurate wavelength scale
for the said sample spectrum. The apparatus is set up for WCM by
sending a particular sequence of commands to the microprocessor 900
using a computer as a terminal. The encoder 600 has a home pulse
which is used by microprocessor 900 to initiate data acquisition.
Apparatus 10 need only be calibrated once for any particular
application. Because slits 105a and 105b are not at the same
spatial location, the spatial location of the said spectra 107a and
107b will be independent. The independent spectra 107a, and 107b
must first be positioned such that the wavelength of the first
spectral peak .THETA..sub.o of spectrum 107b, coincides with a
known reference wavelength of the spectrum 107a. This is done by
dynamically moving the spectrum 107b using the motorized slit 105b
until the first peak signal of spectrum 107b and a known peak
signal of spectrum 107a coincide spatially. The method involves
either an automated calibration routine controlled by
microprocessor 900, or a manual calibration routine, both involving
a fine tuning of the spatial location of the slit 107b. Firstly,
the spectral region of interest is chosen by the user. For example
the user may restrict the spectral range of interest from 200 nm to
1000 nm. A calibration filter with a known wavelength peak
.THETA..sub.c just outside the spectral region of interest is
placed in the path of the light from the said first light source
101 using a good reflector for a sample in port 112 of the
apparatus. Such a filter will have a peak wavelength at say
.THETA..sub.c =198 nm. The calibration method is then carried out
as follows:
a) Continuous Intensity Calibration
The intensity of the reflected sample light and the reference light
source 102 are continuously monitored by the sensitive surface 201b
of detector 201. the ADC 702 continuously digitizes combined
signals from light guides 116a and 116b. The signal values are used
to continuously establish a baseline for the intensity of the
sample spectrum at each wavelength point.
b) Wavelength calibration
The peaks of the said second reference light source spectrum 107b
can easily be identified by microprocessor 900. The intensity of
the reference light source 102 is greater than the intensity of the
first light source 101. Since the spectrum 107d is a superposition
of spectra 107a and 107b (addition of 107a and 107b), the peaks of
spectrum 107b will always occur above a given intensity threshold
I.sub.s. The threshold I.sub.s, is determined by the intensity of
the primary light source 101. Spectrum 107d will have the
pronounced spectral emission peaks corresponding to the wavelengths
of the second reference light source spectrum 107b. These peaks
distinct from the spectral peak of the said sample filter at
wavelength .THETA..sub.c. Using the wavelength .THETA..sub.c as a
fixed reference for the spatial distribution of the wavelength
scale of the sample spectrum.
The relative location of the spectrum 107b of the reference source
can be determined as follows. As the slit 202 exposes the detector
201 to the spectrum 107d of the said reference filter and the
reference light source, an electrical signal is generated by
detector 201. ADC 701 digitizes the said signal at fixed fast
sampling rate R.sub.s. The digitized signals are recorded in the
memory of microprocessor 900 as spectral data. The time difference,
or the number of analog to digital (A to D) counts between
consecutive peak signals is also recorded by microprocessor 900. At
the same time the motor 901 is powered by the microprocessor 900 to
slowly move the spectrum 107b relative to the spectrum 107a, by
moving the slit 105b along Rowland circle 104a. The recorded
differences in times, or the number of A to D counts separating the
peaks for each passing slit 202 is recorded. The time difference or
the number of A to D counts recorded ; between any two particular
peaks of the said reference light source spectrum 107b during the
passage of any two consecutive slits will be almost constant. The
time difference, or the number of A to D counts recorded between
any peak of the spectrum 107b and the said calibration filter
reference peak .THETA..sub.c during the passage of any two
consecutive slits will be changing. The spectrum 107d will
therefore be constantly changing. Microprocessor 900 is programmed
to determine the relative time difference or number of A to D
counts between the first spectral peak at wavelength .THETA..sub.o
of the said reference spectrum 107b, and the calibration reference
spectral peak at wavelength .THETA..sub.c for each slit 202. The
object of the calibration is to control the motion of the motor 901
in order to make the time difference or A to D counts between the
said two wavelengths .THETA. .sub.c and .THETA..sub.o zero, so that
they are spatially coincident on the spectral plane 107c. A feed
back control loop is used to control the motor 901 to move the slit
105b in order to shift the spectrum 107b until the wavelength peak
.THETA..sub.o and .THETA..sub.c coincide on the spectral plane
107c. After this is achieved the microprocessor 900 resets the
apparatus 10 to Sampling Mode (SM).
SAMPLING MODE (SM)
After the (WCM) outlined above, the apparatus is ready for Sampling
Mode. To initiate sample scanning and analyses, the user sends a
set of commands to the microprocessor 900, using a computer as a
terminal. Microprocessor 900 sets the apparatus for Sampling Mode
(SM). The calibration filter is replaced by the sample to be
scanned using either the direct mode or the remote mode of
operation of apparatus 10. Mask 200 is automatically rotated to any
of the said home positions of slits 202a and stops. Sampling mode
operation involves an intensity measurement and a spectrum
measurement. The intensity calibration for the spectrum of the
sampling has been described above. Scanning is initiated by sending
a command to microprocessor 900. The command could specify the scan
rate, the time separating the said scans, the ADC sampling rate,
the names of data files, and the sequence of processing of the data
files. Microprocessor 900 is an integrated computer which includes
a Digital Signal Processor DSP 902 for rapid computation, and rapid
statistical analysis.
As a slit 202a exposes detector 201 to the spectrum 107d, an
electrical signal is generated by the detector element 201 and
digitized by the ADC 701 at a fixed fast sampling rate R.sub.s. The
data is stored by microprocessor 900 in memory. Each data point is
defined by a three parameters:
(a) the sample count of the ADC 701 after determination and
location of the reference peak .THETA..sub.o,
(b) the magnitude of the digitized detector signal, determined from
the intensity of the light measured by detector surface 201a,
(c) the intensity correction factor at that instant in time,
determined by the intensity of light measured by detector surface
201b.
This information is then transferred to the Digital Signal
Processor (DSP) 902 as it is acquired for computation of the
spectrum and rapid analysis of the spectral data. The said DSP 902
is designed to analyze data at a very fast rate using special
programmed algorithms. The computation of the spectrum is as
follows:
(a) The intensity value of the sample light is measured by
sensitive surface 201a of detector 201. this represents the
intensity of a given wavelength of the superimposed spectra.
(b) An initial wavelength reference point .THETA..sub.o is obtained
when the first peak signal is measured by the sensitive surface
201a of detector 201. The said first peak signal represents the
first known wavelength peak .THETA..sub.o of the said second
reference spectrum 107b, and therefore the known wavelength
.THETA..sub.c of the said calibration filter. When the peak value
is obtained, the microprocessor 900 resets the sampling count of
the ADC to zero. The magnitude of each A to D count is recorded
from then on.
(c) The peaks of the said second reference light source spectrum
107b can easily be identified by microprocessor 900. The intensity
of the reference light source 102 is greater than the intensity of
the first light source 101. Since spectrum 107d is a superposition
of spectrum 107a and spectrum 107b, the peaks of spectrum 107b will
always occur above a given intensity threshold I.sub.s. The
threshold I.sub.s, is determined by the intensity "intensity
calibration signal" obtained by surface 201b of detector 201.
d) The sample counts n.sub.o,n.sub.2, n.sub.3 . . . and time
difference between two consecutive peaks of the spectrum 107d is
determined by microprocessor 900. Microprocessor 900 now uses the
known wavelength value of each peak signal of the spectrum 107d as
known wavelengths of spectrum 107b. Microprocessor interpolates a
wavelength scale between the said known peaks for spectrum 107d.
The last known peak signal is used by the microprocessor 900 to
determine the last reference point for the wavelength scale. The
said last signal is also used to stop spectral data
acquisition.
e) the spectrum is determined by microprocessor 900 by using the
"intensity calibration signal" value at each sample point to
compute a base measurement for the "spectral intensity signal" at
the said sample point.
The wavelength scale so generated will primarily be determined by
the spectrum 107b. The interval between the wavelengths of two
consecutive peaks will have a resolution that depends on the speed
of rotation of mask 200 and the sampling rate of the ADC 701. The
resolution of measurement can be increased by slower rotation of
mask 200 or increasing the ADC sampling rate, or a combination of
both. For example the resolution of the wavelength scale can be
doubled by reducing the speed of rotation of mask 200 by a factor
of one-half.
In this embodiment of the invention, the microprocessor 900 is used
to set the sampling rate of the ADC and the rotational speed of the
mask 200. Motor 300 has a resolution of 50000 or more steps per
revolution. This ensures that the stepper will resolve the
wavelength scale into at least one-half nanometer per step.
Mask 200 rotates at a uniform speed of about 60 revolutions per
second, so that 720 scans can be acquired per second can be
acquired. The sampling rate of the ADC is set at about 200 MHz.
If "m" samples are taken from the wavelength interval spanning two
consecutive peaks .THETA..sub.c and .THETA..sub.1, the wavelength
resolution will be
The spectral intensity at each sample count taken between and
.THETA..sub.c and .THETA..sub.1 will be
By knowing the absolute value of .THETA..sub.c (same wavelength as
the calibration filter peak wavelength) the spectrum 107d can be
mapped into the wavelength scale generated by the described method.
This process is repeated for all the consecutive peaks of the said
reference spectrum, thereby obtaining a continuous but dynamic
wavelength scale of great accuracy for each scan. Each scan is
completed when a slit 202a passes the last known spectral peak of
the spectrum 107d. The process is repeated for each of the 12 slits
202a. Each passing slit 202a will produce an accurate spectrum of
the sample in the wavelength region of interest.
Since the number of interpolated points on the wavelength scale is
a function of the sampling rate of the ADC, the resolution of the
method is theoretically limited only by the optical resolution of
the apparatus. However in practice, the finite time constants and
responsitivity of the said detector element 200 and the stability
of the said second reference light source will limit the accuracy
of the method. This is also true for any other known method of
spectral data acquisition.
The accuracy of the wavelength calibration method disclosed herein
also depends on the closeness of the spectral peaks of the said
second reference light source and the accuracy of the initial
calibration routine. The novelty of the wavelength calibration
method disclosed in this invention lies in the fact that no encoder
is needed to define the wavelength scale. The detector 201 and the
spectrum 107b of the reference are used to perform the functions of
an encoder. The speed of the motor 300 need not be very accurate,
since the disclosure reveals a method measuring the speed at
different regions of the spectrum, and compensating for speed
variations at different regions of the spectrum.
The novelty of the intensity calibration method disclosed lies in
the fact that the method completely eliminates any ambient light
character from the measurement at each wavelength. The method also
eliminates any light source variations in the apparatus, or outside
the apparatus. It is thus a very suitable method for process
instrumentation design.
FIG. 10 shows graphs of the superimposed reference and sample
spectrum and the extracted sample spectrum respectively.
The sample spectrum S.sub.a can be extracted from the spectrum 107d
by taking a reference background spectrum S.sub.r of the system
with no sample in the path of the light from the said first source,
and subtracting this from the spectrum 107d. The final spectrum of
the sample is obtained by subtracting the baseline S.sub.b of the
intensity calibration signal.
In principle, it is possible to compensate for detector slew rates,
and responsitivity using the method here disclosed, since the time
constant associated with the measurement of the wavelength scale
using the reference spectrum 107b also apply directly to the said
spectrum 107d.
The method of wavelength calibration disclosed in this invention
allows the wavelength scale to be dynamically imprinted on detector
element 201 at each and every scan cycle of apparatus 10. Because
there is no time difference between a sample measure scan and a
wavelength reference or calibration scan, the wavelength scale
generated for the said measurement is very accurate. Also the
potential errors introduced by using an encoder 600 to determine
the spatial location of the said rotating mask 200 are reduced by
the present invention, since the wavelength scale is ultimately
determined by the dispersion of the spectrum 107b of the said
second reference light source 102 and not by an independent entity
such as encoder 600. A variation in the properties of grating 106
for example, cannot affect the said wavelength scale generated by
the method here described, since such errors are automatically
corrected by interpolation between the known spectral peaks of the
sample.
Almost all existing instruments, (moving grating or stationary
grating) use methods independent of the spectrum itself to
determine the wavelength scale for both the sample spectra and the
reference spectra. For example instruments using static diode
arrays depend on the spacing of the detector array and the spectrum
to determine the wavelength scale. A detector with N elements
spanning a wavelength region of L nanometers divides the wavelength
scale into L/N segments. Variations in L will affect the wavelength
scale of such instruments. Moving gratings on the other hand use
optical encoders to define the wavelength scale. Relative motion of
the encoder shaft and the moving grating mass results in wavelength
scale errors. Furthermore, the present invention accounts for
errors due to non-linearity of the spectrum as a result of thermal
and mechanical stresses on the grating of the spectrograph and
detector substrate. These errors are automatically corrected by the
said method of interpolation discussed above. Non-linearity can
manifest in varying degrees at different regions of the said
spectrum 107d. The interpolation routines between fixed known
wavelengths of the said spectrum of the said second reference light
source should minimize these effects.
* * * * *